Fatigue Crack Propagation Behavior and Fractography in Hardness Heterogeneous Field

Abstract

The fatigue crack propagation path is not always straight nor vertical to the stress axis, when a hard region lying ahead of the crack tip is inclined to the fatigue crack propagation direction.
In this report, the effect of such hard region to the propagation direction and the mechanism for the change of fatigue crack path were investigated using the specimens having a plain soft-hard boundary which was induced by induction hardening.
The fatigue crack got inclined toward the relatively softer side and propagated in the soft region parallel to the soft-hard boundary. The residual stress seemed not to have much effect on the change of propagation direction but the hardness heterogeneity had a marked effect on it.
The fractographic observation revealed the symmetric striation at the place where the crack propagated straight, and the asymmetric striation and many secondary cracks growing out of the fissures at the places where the crack inclined. From these findings, it seems that the change of propagation direction is caused by the formation of the asymmetric striation and the micro crack branching.